I did this, on a smaller scale. The house I live in was built on an old orchard and we were graced with one of the very few remaining apple trees in the garden. It produces relatively sour apples useful for cider and cooking etc. We think they are Bramleys but not 100% sure. I grafted a few branches off a Blenheim Orange stock a friend had that had fruited for a couple of years first. 2 didn't work but 4 did and we have two varieties now on one tree. They're in the same pollination group as well and we planted meadow flower mix around it to attract the pollinators so you actually end up with more apples on both parts of the tree. Last year one of the branches snapped off entirely because there were so many apples on it.
Edit: photo from a couple of years back after we nearly emptied it on a cider run :) - http://i.imgur.com/aYacpT4.jpg
It's quite easy, perfectly routine and has been done for a very long time indeed. Hundreds of years if not more.
You can take this to extremes if you really want (sorry for the Daily Mail link): http://www.dailymail.co.uk/news/article-2437247/250-varietie...